1. Technical Field
The present invention relates to a semiconductor integrated circuit device and a method of driving the same, and more particularly, to a magnetic memory apparatus having multi-levels and a method of driving the same.
2. Related Art
Along with high speed and low power consumption, a fast write/read operation and a low operation voltage are also useful characteristics of memory devices embedded in the electronic appliances. Magnetic memory apparatuses have been suggested to satisfy the useful characteristics. Magnetic memory apparatuses have high speed operation and/or non volatile characteristics.
In general, magnetic memory apparatuses may include magnetic tunnel junction patterns (hereinafter, referred to as MTJs). The MTJ may include two magnetic materials and an insulating layer interposed between the two magnetic materials. A resistance of the MTJ is varied based on magnetization directions of the two magnetic materials. More specifically, when the magnetization directions of the two magnetic materials are anti-parallel to each other, the MTJ may have a large resistance, and when the magnetization directions of two magnetic materials are parallel to each other, the MTJ may have a small resistance. Therefore, the MTJ may have different resistances, and it is possible to read/write data according to a resistance difference.
Some magnetic random access memories (MRAM) have created an MTJ device by forming a ferromagnetic tunnel junction as a magnetoresistance device. The MTJ device includes three-layered layer including a magnetic layer, a nonmagnetic layer, and a magnetic layer and current flows to tunnel the nonmagnetic layer (a tunnel barrier layer). Another MTJ device design, called a spin valve structure, contains an antiferromagnetic layer that is disposed adjacent to a magnetic layer, and a magnetization direction is fixed to improve a sensibility of a magnetic field.
In the MRAM, a magnetization state of a ferromagnetic material forming a unit cell may be changed by a magnetic field. Alternatively, there are current-induced magnetoresistance devices where a magnetization state of a ferromagnetic material is changed by applying a current. The current-induced magnetoresistance devices are devices that control a magnetization direction by applying current to a magnetic layer to control a magnetization direction of the magnetic layer.
A method of reading information from the current-induced magnetoresistance device is similar to a magnetic-induced MTJ or giant magnetoresistance (GMR) devices. When a relative magnetization direction of a free magnetic layer and a fixed magnetic layer is parallel, the device has a low resistance. On the other hand, when a relative magnetization direction of the free magnetic layer and a fixed magnetic layer is anti-parallel, the device has a high resistance. Therefore, the resistance states of the device may correspond to digital values “0” and “1”.
Currently, a dual GMR structure has been suggested to obtain multi-levels other than “0” and “1” (J. Appl. Phys. 105, 103911, 2009).
As shown in FIG. 1, the dual GMR structure includes a first fixed layer 10, a first spacer layer 20, a free layer 30, a second spacer layer 40, and a second fixed layer 50. The first and second fixed layers 10 and 50 may include a ferromagnetic material having a fixed magnetism. The free layer 30 may be an antiferromagnetic material having a magnetism that varies according to an external magnetic field. The first and second spacer layers 20 and 40 may be formed of a CoFe/Cu/Co material.
When the magnetism of the first and second fixed layers 10 and 50 and the free layer 30 are changed to implement multi-levels, the dual GMR can implement only three levels, (0,0), (0,1) and (1,0), as shown in FIG. 2.
Therefore, a magnetic resistance memory apparatus capable of implementing more various levels is desired.